Boon Geuze Mariage Parfait consists of 95% mild lambic, aged at least three years and specially reserved for this purpose, and 5% young lambic. The latter provides fermentable sugars and wild yeasts. After mixing in a vessel of 25,000 liters, the wort (unfermented mixture) is filtered and chilled. If we are bottling, we bring it back to fermentation temperature. The bottles are placed in a climate-controlled space for the secondary fermentation. This is followed by maturing at a low temperature for at least 6 months. Want to know the bottling date? Simply subtract 20 years from the “best before” date!